This invention relates to optical lithography and more particularly to an improved apparatus for projection printing with imaging optics in the production of printed circuits and the like.
Photoresist exposure, in the integrated circuit industry for example, is accomplished by contact printing with collimated light or by projection printing with imaging optics. In contact printing procedures, a light source is incident on a lens system designed primarily to collimate the source. The collimated source exposes a resist-coated wafer or substrate through a patterned mask with which the wafer is in contact. With a projection system, the mask and wafer are physically separated. The function of the lenses in the latter system is to project an image of the illuminated mask onto the wafer surface. Projection printing is particularly popular for step-and-repeat processes, eliminates mask wear, and relaxes mask resolution requirements when minifying optics are employed.
Since projection lenses are usually designed to yield state-of-the-art performance for geometrical distortion and resolution, they operate over a restricted spectral range, such as a single filtered strong line from light source such as a mercury arc lamp. This produces optical standing waves in the resist layer during exposure resulting in a layered resist structure. In addition, there is critical dependence of required exposure time on the absolute resist thickness thereby necessitating tight control of resist thickness.
This invention is directed to apparatus for projection printing on photoresist layers which eliminate standing waves in the layer.